Variable area magnetic recording apparatus



R. B. ATKINSON ET AL 2,853,560 VARIABLE AREA MAGNETIC RECORDINGAPPARATUS septzs, 1958 3 Sheets-Sheet 1 Original Fled Jan. 16, 1952Sept. 23, 1958 R. B. ATKINSON ET AL 2,853,560

- VARIABLE AREA MAGNETIC RECORDING APPARATUS Originanl Filed Jan. 16,1952 I 3 Sheets-Sheet 2 lNDUCTlON I zeslDuAL pas MAGNETIZWG FORCE l O Y'Xga? 8 'l2 INVENTORS I 5 @nm/l B. Afr/anso 87 .5r-EVEN G. ELL/s I Sept.23, 1958 R. B. ATKlNSON ET AL 2,853,560

l VARIABLE AREA MAGNETIC RECORDING APRARATUS Original Filed Jan. 16,1952 3 Sheets-Sheet 5 INVENTORS Enum B. #rz/50N 'Sn-VEN G. ELL/5 52MWMM@ l DDDDUCIUUUCIDDD UDUDUODDUDDD CIUUDUDdUDUUD` VARIABLE AREAMAGNETIC RECORDING APPARATUS Ralph B. Atkinson, Los Angeles, and StevenG. Ellis, Van Nuys, Calif.; said Ellis assignor to said AtkinsonOriginal application December 15, 1951, Serial No. 261,892, now PatentNo. 2,806,904, dated September 17, 1957. Divided and this applicationJune 29, 1954, Serial No. 440,174

8 Claims. (Cl. 179-1002) This invention relates generally to themagnetic recording of intelligence, and has particular reference to amagnetic recording apparatus .for producing a vari-able area magneticrecordingof signals corresponding to such intelligence.

This application is a division of our copending application Serial No.261,892, tiled December l5, 1951, for Variable Area Magnetic RecordingApparatus.

Magnetic recording on a specially designed magnetic tape or wire haslong been known and used, and many advantages yof such recording havewidened the field of use considerably. More recently, the utility of themagnetic recording processes has been greatly increased by thedevelopment of certain methods for making visible magnetic recordings asdistinguished from the earlier types yof recordings wherein the recordedmaterial was completely invisible.

It is possible to render the magnetic recording visible by liowingfinely divided magnetic particles over the magnetic recording so thatthe particles lare held to the record material in varying amountsdepending upon the intensity of magnetization of the particular area ofthe material. A method and apparatus for magnetic reproduction ofpictures using this general method vof flowing magnetic particles over amagnetized record is the subject of a copending application Serial No.221,044, tiled April 14,

1951, by Ralph B. Atkinson and Steven G. Ellis. As is p disclosed inthat application, the visible record formed upon the m-agnetized recordmaterial may be transferred to another support to produce a permanentprint or copy of the magnetic record. The process of making visiblerecords from magnetic records has been termed ferrog-y raphy, and theprocess has come to be known as the ferrographic process asdistinguished from the photographic process wherein light-sensitivematerials are ernployed.

In a copending application Serial No. 250,364 led October 8, 1951, byRalph iB. Atkinson and Steven G. Ellis, it is pointed out that prior tothe invention thereindisclosed of an apparatus for making a variablearea magnetic recording, ferrography had seldom been used in therecording of technical data because the resulting visible recordrevealed only areas of dilerent densities, because prior thereto themagnetic recording apparatus in use was capable lonly of producing avariable density record. Since it is diiiicult to evaluate differencesin intensity, either -by observation or physical measurement, the use ofthe ferrographic process for the recording of technical data was limitedahnost entirely to those applications where it was desired only toobtain qualitative results.

The aforementioned copending application Serial No. 250,3 64 discloses`an apparatus for producing a variable area recording wherein the heightof the record at any given point corresponds to the intensity of therecorded signal. By making such a record visible by the ferrographicprocess, it is possible to yobtain a record comprising an oscillogram ofthe applied signal. Such a recordcombines the advantages of a recordmade by a l atent O "ice l?. direct-reading, pen-type oscillograph withthe speed of response corresponding to that of the cathode rayoscilloscope. ln addition, the disadvantages of the photographic method-of recording encountered in both the mirror-type oscillograph andcathode ray oscilloscope are avoided.

In the apparatus disclosed in the -aforementioned copending applicationSerial No. 250,364 a variable area recording is obtained by passing therecording medium beneath a recording head which provides a recording gapof substantial length extending transversely of the direction of travelof the recording medium. In one lform `disclosed in that application,the recording gap is made V-shaped-that is, wider at one end than at theother, thus -causing a concentration of the ux at the narrow end of thegap. Thus, at low signal intensities a permanently magnetized record isproduced only in that portion of the recording medium passing the narrowend of the gap, Whereas at higher signal levels, the iiux density isincreased across the wider portions of the gap so as to record acorrespondingly wider line lon the moving recording medium.

While the apparatus disclosed in said aforementioned copendingapplication Serial No. 250,364 is entirely practical, the apparatusnevertheless embodies certain `disadvantages residing principally in thedifficulty of forming a V-shaped gap `of the required accuracy. It Willbe appreciated that the gaps used in magnetic recording are extremelysmall, and a V-shaped gap for producing a variable area magneticrecording may have a minimum width of one-half mil (the term mil beingused to represent a `distance of one one-thousandth of an inch), and amaximum width of perhaps one and onehalf mils. Furthermore, in order toachieve a desired recording characteristic, it may be necessary to makethe sides of the gap curved 4as distinguished from the straight sides`of a truly V-shaped gap. The use of a curved support for a variableclearance between the recording head and the recording medium disclosedas alternativey modications in said copending application Serial No.250,364 does not completely obviate the disadvantages and difficultiesof construction of the V-shaped -gap because they introduce the diicultyof feeding the recording medium over a curved support and also tend toproduce loss of definition due to fringing resulting from the spacing ofthe recording head from the recording medium.

To overcome the above-mentioned disadvantages, this invention provides avariable area recording apparatus utilizing a recording gap ofsubstantial length and uniform width, and eliminates the need for anycurved supports for the recording medium or the necessity of spacing therecording head from the record material. Instead, the present inventionprovides a recording apparatus which is maintained in constant contactwith the recording medium throughout the entire length of the elongatedrecording gap, selection of substantially any desired relationshipbetween the location along the length of the gap and the magneto-ymotive force across the gap at that location.

lt is therefore an object of this invention to provide a magneticrecording apparatus for producing variable area recordings whichutilizes a recordinggap of substantial length and uniform width which ismaintained in contact with the recording medium throughout the length ofthe recording gap.

It is also an object of this invention to provide an apparatus of thecharacter set forth in the preceding paragraph which includes anelectromagnet for producing an integrated magnetomotive force across therecording gap as a whole, corresponding to a signal to be recorded,together with means for producing a concentration of magnetic tluxacross the gap at one end thereof.

and provides also for the It is another object of this invention toprovide an apparatus of the character set forth in the precedingparagraph which includes a magneitc circuit external to saidelectromagnet in which said circuit includes .said recording gap andconsists of a plurality of .parallel branch circuits crossing said gapat different locations, the reluctance of said branch circuits. beingafunction of the location at which said gap is crossed thereby.

It is an additional object of this inventiontoprovide an apparatus ofthe character set forth hereinbefore.. in which means is providedA forproducing across the recording gap a gap magnetomotive force bearing aratio ,to the integrated magnetomotive force which isa function of thelocation along the length ofthe gap.

It isnalso an object of this invention to provide -a magnetic recordingapparatus for producing variable area recordings which includes anelectromagnet for production an integrated magnetomotive force acrossthe recording gap as a whole, correspondingto a signal to be recorded,and which includes a pole structure comprising a pair of adjacent polemembersv separated by arecording gap of substantial length and uniformWidth, together with a magnetic circuit means connecting opposite polesof said electromagnet to the ends of corresponding pole members, saidpole members having a reluctance from end to end comparable to thereluctance of said recording gap.

It is another object of this inventionto provide an ap` paratus of thecharacter set forth in the preceding paragraph in `which the reluctanceper unit lengthof said pole members varies along the length thereof.

It is a still further object of this invention to provide an apparatusof the character set forth inthe preceding paragraphs in which the areaper unit length of said recording gap varies along the length thereof.

Other objects and advantages of this invention will be apparent from aconsideration of the following specification, readin connection with theaccompanying drawings, wherein: l

Figure l is a perspective view illustrating one form which a variablearea recording apparatus constructed in in accordance with thisinvention may take;

Figure 2 is a perspective view illustrating the construction of avariable area recording head embodying the principles of this invention;

Figure 3 is a diagrammatic view representing the magnetic circuitsemployed in the structure shown in Figure 2;v

Figure 4 is a schematic diagram representing an electrical analogy tothe magnetic circuit arrangement of Figure 3;

Figure 5 is a graph representing the manner in which the iiux densityacross the recording gap at a given location Varies as a function ofthat location along the length ofthe recording gap;

Figure 6 is a graph representing the magnetic characteristics of typicalmagnetic recording media;

Figure 7 is a diagrammatic representation of the type of record producedby a recording head of the character illustrated in Figure 2;

Figure 8 is a graph illustrating the wave f orm of the signalrepresented by the recording illustrated in Figure 7;r

Figure 9 is a fragmentary sectional view of a modified structure similarto Figure 2,. Figure 9 representing the construction of the modifiedapparatus when viewed in a horizontal section such as might be takenthrough the horizontal pole members of Figure 2;

Figure l0 is a fragmentary perspective view of an apparatus similar tothat shown in Figure 2, but illustrating -a further modified form ofconstruction;

Figure 1l is a longitudinal sectional View of a still further modiiieddevice, the section of Figure ll being a vertical section such as mightbe taken through the spacer member between the horizontal `pole. membersof-Figure 2;

Figure 12 is a cross-sectional view of the device shown in Figure 11,Figure 12 being taken substantially along the line 12-12 of Figure 11;

Figure 13 is a diagrammatic View illustrating the appearance of atypical oscillogram produced through the use of the apparatus of thisinvention, Figure 13 illustrating the-"frequency response characteristicof an amplifier;

Figure 14 is a diagrammaticl representation of the form of recordproduced by an apparatus of the type shown in Figure 15 when there isapplied thereto a signaliof the character represented by Figure 8; and

Figure 15 is a perspective view of a still further modication of theinvention, designed to produce a somewhat different type of recordingfrom that produced by the devices illustrated in the preceding figures.

Essentially, this invention contemplates the use of a magnetic tapetransport means, a magnetic recording head of special constructionagainst which the tapeV bears andacross which it passeswhile beingtransported, together with associated electricalapparatus, for supplyingan` appropriate signal to the recording-head. Such an apparatus isillustrated in Figure 1 as comprising a cabinetor housing which enclosesa suitable electric motor (not shown) or other source of'motive powerfordriving film sprockets or rollers 31 for movinga transportable magnetictape 32 past a recording head 33. As the tape 32 is moved past therecording head 33, it is maintained in constant andfintimate Contacttherewith by a roller 34 orpressure padof usual and conventionalconstruction.

To facilitate transporting the tape 32.1'n the manner described, thesame is prefera'blyperforated in thegeneral manner `of motionppictureAfilms. While unperforated tape may be used, and while the tape 32 mayrbe of substantially any desired width, a ,preference is expressed for atape having the widthy andfsprockethole arrange-y ment of conventional35 mm. iilrn becauseof the ready availability of suitable sprockets,reels, and the like.

The drive means for rotating the sprockets 3 1is arranged preferably totransport t-he tape at a constant linear speedpast the recording head 34so as to permit ready`and accurate measurements with respect to time.When soitransported, the tape 32` is fedfrom` at suitably supportedsupply reel 35, and after recording. is wound upon a suitable take-upreel36.

The recording head 33, whichis of-special construction, may beconstructed in accordance with the prin-v ciples-,illustrated in Figure2 to include. an electromagnet 40, a pair of pole-members 41-.andV42wand :magneticzcircuit means 43 and 44 for connecting opposte;poles'of' the electromagnet 40 to adjacently disposed endsiofthe polemembers41 and 42, respectively.

At the outset, it should be recognized that the drawings accompanyingthis application lare :largelydiagrammatic, the arrangement of parts anddetails of construction shown having been selected with ,aview'to bestillustrating the principles of the invention. Different arrangements anddetails of construction which will ,be= obvious to thoseskilled intheartmay befemployedtto embody the principles of this invention in morecompact structures designed also to vfacilitate.themanufacture thereof.Furthermoreitnshould :be recognizedthat thefdrawings are not to scaleandare=malproportioned for the purpose of clearly illustrating the:essential features of construction, gaps and certain structural IpartsAhaving widths and thicknesses ofthe order of magnitude of one mil beingrepresented in the.drawings as being of substantial width rand thicknessfor the purpose of clearly illustrating the construction. l

The electromagnet 40 may bevof substantially conventional construction,wound to provide a number 'of' turns suicient to produce a-substantialtmagnetomotive force vand to providezanimpedanceappropriatefto the source of signal toberecorded. The magnettcirc-uitmembers 43 and 44 may :alsobefconstrueted along'conventional lines',preferably being laminated to minimize hysteresis and eddy currentlosses, and are preferably made of a high-permeability, low-lossmaterial. The members 43 and 44 are proportioned to provide a reluctancewhich is low compared to the reluctance of the pole structures 41, 42and low compared to the reluctance of the recording gap.. The magneticcircuit members 43 and 44 are connected as shown to one end of each ofthe pole structures 41 and 42 so as to provide between said ends amagnetomotive force which is substantially equal to that appearing atthe poles of the electromagnet 4b.

The pole structures 41 and 42 are secured in side-byside relation toeach other to define therebetween a recording gap 45. The recording gap4S is made of uniform width, preferably of the order of magnitude of onemil, and is made with a length (equal to the length of the polestructures 41, 42) appropriate to the width of the recording medium 32.When used with 35 mm. film,

the recording gap may have a length of about one inch. l

A recording gap 45 of uniform width throughout the length thereof ismost easily lobtained by the use of a metallic spacer member 84 having athickness equal to the desired gap width and formed of a suitablenonmagnetic material such as brass or beryllium copper. Such sheetmaterial is readily available in a form sometimes known as shim stock ina Wide range of thicknesses ranging upwards from one-half mil. Suchmaterial is eminently suited to use as the spacer member 84 because `ofits ready availability and, what is more important, because of the greatuniformity of its thickness and the accuracy with which the actualthickness of the material corresponds to its nominal thicknessdimension.

The pole structures 41 and 42 comprise pole members 81 and 82 which areso constructed as to have an appreciable reluctance. This may beaccomplished by suitably proportioning the 'cross-sectional area of themembers, and by a suitable selection of the material from which themembers are made, such selection being made with reference to themagnetic properties of the material.

The manner in which the apparatus illustrated in Figure 2 operates toprovide a variable area magnetic recording may perhaps be bestunderstood by having reference to Figure 3 which illustratesdiagrammatically a magnetic circuit approximately equivalent to that ofthe apparatus shown in Figure 2. In Figure 3 each of the pole members41, 42 is illustrated as consisting of a plurality of low reluctancemagnetic elements 47 and associated gaps 48 separating each member 47from the next adjacent such member. The reference letters a-f of Figure3are used herein as suixes to denote the relative locations of thevarious parts of the magnetic circuit. Thus, 47C represents the magneticmember 47 at location c, and similarly 45b represents the recording gapbetween oppositely positionedmagnetic members 47 at location b. It willbe seen that the pole structures 41 and 42 of Figure 3 approximate themagnetic properties of the actual pole structures shown in Figure 2, thetotal reluctance of the gaps 48 being equal to the total reluctance'fromend to end of the actual pole structures. The closeness of theapproximation depends on the number of discrete elements assumed,approaching full equivalency, the approximation is suiciently close topermit an accurate explanation of the mode of operation.

Because of the low reluctance of the magnetic circuit members 43 and 44,the full magnetomotive force generated by the coil 40 may be consideredas applied between the tirst two magnetic members 47a. It will be seenby an inspection of Figure 3 that this full magnetomotive force existsacross the first portion 45a of the recording gap. It will be equallyapparent that the flux density across .the recording gap section 45t:will be less than that across the gap section 45a, because in additionto overcoming the reluctance of the recording gap 45h, such ux must-alsoovercome the reluctance of the two gaps 48a in the pole members 41 and42, respectively. Similarly, it will be seen that the flux densityacross a more remote recording gap section such as the section 45d, forexample, will be still further reduced, since the flux passing acrossthat portion of the recording gap must overcome not only the reluctanceof the gap section 45d, but also the reluctance of the gaps 48a, 48h,48e of the pole member 41, and of the gaps 48e, 48h, and 48a of the polemember 42.

An analogous electrical circuit is illustrated in Figure 4, in which thesource of magnetomotive force is replaced by the battery E providing anelectromotive force. The electrical conductors are analogous to theportions of the recording head which are made of low reluctance magneticmaterial, and the resistances Rrr-Re correspond to the air gaps48a--48e. The cross connections Rga-Rgf correspond to the reluctances ofthe air gaps 45a-45f. In Figure 4 it will be seen that the fullelectromotive force E is applied across the rst resistance Rga. Asomewhat smaller voltage is applied across the second resistance Rgb dueto the voltage drops in the two resistances Ra produced by the currentpassing through the cross-connected resistances Rgb-Rgf- In like mannerthe voltage across the resistance Rgc is less than that across theresistance Rgb due to the voltage drops through the resistances Rbproduced by the sum of the currents drawn by the cross connectedresistances Rgc-Rgf.

In Figure 5 there is plotted a family of four curves 51, 52, 5'3, and 54representing the flux density, or alternatively the magnetomotive force,existing across the recording gap 4S as measured at different positionsalong the length of the gap, the origin of the curve being taken at theinnermost end of the pole members 41 and 4- that is, at the ends thereofwhich are connected directly to the magnetic circuit members 43 andv 44.The four curves 51-54 represent the ilux densities produced by fourdifferent intensities of magnetization produced by the coil 40. Thehorizontal dotted line 55 represents a threshold level of gapmagnetomotive force below which no permanent magnetization of therecording medium will be produced. Curve 51 therefore represents theminimum signal level which, when applied to the coil 41D, will produceany permanent magnetic record, and by observing that the intersection ofthe curve 51 with the line 55 occurs at the origin, it will be seen thatthis represents the limiting condition producing a permanent record onlyat the extreme edge of the tape. Curve 52 represents a higher level, andit will be observed that the curve crosses the line 55 at the pointmarked 56. Thus in the region to the left of the point 56 themagnetonrotive force across the gap will be sufficient to produce apermanent magnetic record, whereas in the region to the right of thepoint 56 the magnetizing intensity will be insulcient. Thus a signal ofa magnitude such as is represented by the curve 52 will cause apermanent recording to be made for a distance transversely of the tapeapproximating one-third the length of the gap.

The curves 53 and 54 respectively intersect the thresholdlevel line 55at the points marked 57 and 58, representing, respectively, theintensities of magnetization ret quired to produce permanent magneticrecordings extending, respectively, for two-thirds the length of the gapand for the full length of the recording gap.y

Whether or not a given magnetizing force will produce a permanentmagnetic record and the intensity of magnetization of that recorddepends, of course, upon the magnetic characteristics of the recordingmedium. There is illustrated in Figure 6 a characteristic curve which isrepresentative of the magnetic characteristics of magnetic recordingmedia generally. In Figure 6 there is shown a curve 60 which representsthe relation between the magnetizing force applied to a magneticrecording medium and the resulting residual induction or permanentmagnetization after the magnetizing force is removed. Assuming a neutralrecording medium, the curve starts at the origin representing zeroresidual induction when no 7 Y magnetizng force is applied. The curverises slowly to a point marked 61f, the abscissa of which is representedby-the dimension line bearing the` reference character62, and theordinate of which is represented by the dimension line bearing thereference character 63. The dimension 63 represents thethreshold'residual induction and constitutes vthat degree of permanentmagnetization which will produce a definite though `low'intensity ofpermanent magnetism in the magnetic recording medium. The dimension line62'represents the amount of magnetizing force which must be applied `toproduce such a threshold condition.

From the point 61 the curve rises steeply to a high value of residualinduction, and 'then curves to the right with a decreasingv slope,finally becoming parallel to the horizontal axis representing the mwimumpossible residual linduction which can ,be produced regardless of themagnitudev of the rnagnetizing force.

The curve 60 is similar to the H and D curve used to represent the,properties of photographic materials, the dimension* 62 correspondingto the inertia of photographic materials, and the slope of the steeplyrising portion corresponding to the gamma of the photosensitivematerial.

Ideally, the magnetic recording medium selected for use with a variablearea magnetic recording head of the type herein described has a smallinertia-that is, a short dimension line 62, so that the amount ofmagnetizing force necessary to produce a permanent record need not betoo great. The steeply sloping part of the curve 66 should have as steepa slope as possible so that once the magnetizing force exceeds thethreshold value, the residual induction will rise rapidly with Asmallincreases in magnetizing force so as to produce a record which inappearance, after being coated with the ,visible magneticv particles,displays a clean, well-dened line of demarcatio'n between those portionswhich have been permanently magnetized and those which have not.-

In Figure 6 the dashed line 55a passing vertically through the point 61represents the threshold value of magnetizing force which is representedin Figure by the dashed line 55.

Thus it is seen from a comparison of Figures 5 and 6 that as the energysupplied to the magnetizing coil 40 is increased, no permanent magneticrecord is produced until the magnetizing force reaches the mentionedthreshold value for the portion of the recording gap 45 nearest themagnetic circuit members 43 and 44, such level of magnetizing forcebeing represented by the curve 51 of Figure 5. Thereafter, as the energysupplied to the coil 4i) isiincreased, the length ofl that portion ofthe recording gap 45 across which the magnetizing force is greater thanthe threshold value correspondingly increases until, at the maximumsignal condition, the magnetomotive force across the gap 45 is at allpoints along the length thereof higher than the threshold value.

Figure 7 illustrates the type of record produced by the describedoperation of the magnetic recording head shown in Figure 2 when there isapplied to the electromagnet 40 thereof a signal of the characterillustrated graphically in Figure 8.

The signal represented by Figure 8 constitutes an alternating electricalpotential of a given fundamental frequency and carrying a substantialsecond harmonic componentl resulting in the production of two distinctpositive peak signals 64 and 65 followed by two corresponding negativepeak signals 66 and 67. Thereafter the cycle repeats itself at thefundamental frequency. The magneticrecord which is produced is of thecharacter illustrated in Figure 7 consisting ofl the positive peaks 64and 65 followed by what may be termed a rectified representation of thenegative half of the cycle resulting in the peaks 66' and 67. Theapparent polarity reversal results from'the fact that themagnetic-material which is flowed acrossthe magnetizedrecordingmediumadheres equally well to both polarities ofmagnetization, anddoesnot'distinguishbetweenthose 'elemental areas having a north polarityand those having a south polarity. Once this fact is recognized, 'thewave form of the original signal is quite clearly seen by yinspection ofthe record produced. The important consideration isthat the length ofthe visible recording, measured upwardly in-Figure 7 fromra base line68v (corresponding to that end of the gap 45 situated nearest themagnetic circuit members 43 and 44)- correspondsV to the instantaneousamplitudes of the signal applied to the electromagnet;

The record produced is represented in Figure -7 as being characterizedby narrow blank spaces 69 corresponding to the points at which thesigna-l level is below the aforementioned threshold value. This isconsistent with the initial assumptions made thus far in describing theoperation of the apparatus'in which a neutral magnetic recordingmediumis assumed, and in which the signal is applied directly to theelectromagnet 40 vwithout the use of any `bias currents or modulatedcarriersignals. These blank spaces` are readily eliminated through' theapplication of any of a number of well-known magnetic recordingtechniques such as the use of a premagnetized recording medium, the useof either direct or highfrequency bias currents, or by the use of acarrier signalI of high frequency upon which the signal to be recordedis applied as an amplitude modulation.

Preferably, the recording head is So constructed as to produce a linearrelationship between the amplitude of the applied signal and the lengthof the recording gap which develops a magnetizing force in excess of thethreshold value. If lsuch a linear relationship is established, then thevisible record produced representsftruly thev wave form or amplitudevariations of the applied signal.

As will be apparent to those skilled in the art, this does notnecessarily require a linear relation between thel magnetizing force andthe distance along the gap at whichl that magnetizing force .ismeasured. In fact, as may bel seen from an inspection of Figure 6, thecharacteristic is most probably curved so as to be concave upwards, butthe degree of curvature and the shape of the curve will depend upon manyfactors, not the leastimportant of which are the characteristics oftherecording medium used.

An upwardly concave characteristic is inherent in the structure shown inFigure 2. By resort to the analogous'- electrical case represented byFigure 4, it can be shown that if the reluctance of each of the gaps 48is made equal to each other and equal 'also to the reluctance oftherecording gap 45, a curve form which is concave upwards, and which isvery sharply curved near the beginning, results. If the effects of strayelds, fringing, and saturation are ignored, the flux distribution acrossthe gap ywill be such that the ux across each gap section 45a, 45b, etc.will be approximately one-fourth that across the nextpreceding section.'A curve having such a great curvature would quite likely fail to producethe desired linear relationship between the amplitude of the inputsignal and the length of the recorded line. However, in practice itv isfound that the saturation and fringing effects cannot be ignored, andthat these effects tend to atten the curve considerably. It can be seen,for example, that as the magnetizing force across the gap 45a increaseswith a corresponding increase in iiux density, those portions of themagnetic elements 47 immediately adjacent the gap 45 will becomesaturated, with a corresponding decrease in permeability and acorresponding increase in reluctance, whereas those portions spaced fromthe lgap will not be so saturated. This tends to divert the flux fartheralong the pole piece, and so causes the flux across each of the gapsections 45a, 45h, etc. to be a larger fraction of the flux densityacross the next preceding gap section than the simpliedcalculationswould indicate.

Adjustment of the shape of the characteristic curve may 9 be made byvarying the cross-sectional area of the members 81 and S2, for exampleas by tapering the members in thickness as is indicated by the members81a and 82a of Figure 9. Such a tapering of the members provides areduced reluctance at the inner end of the structure, and an increasedreluctance at the outer end, thus tending to shift outwardly thethreshold value of magnetomotive force produced across the recording gapby a given magnetomotive force generated by the electromagnet 40.

Somewhat similar results may be achieved by tapering the pole members inheight as is represented by the pole members 81b and 82h of Figure 10.

Figures 11 and 12 illustrate still another way in which a characteristiccurve of desired form may be achieved. In the modification shown inFigures 11 and 12 a pole structure comprising the pole members 81C and82C separated by the spacer member 84C are first produced which aresimilar in all respects to that shown in Figure 2. Thereafter, the uppersurface of the members 81 and 82 is milled away to provide a slot orgroove 86 extending downwardly from the upper surface of the members 81Cand 82e, the depth of the slot 86 being varied along the length of themembers to provide a curved surface 87 such as is shown in Figure 11defining the bottom of the slot.

Assuming that the recording gap 45 is one mil Wide and that the members81e and 82o have a horizontal thickness of about 2() mils, it is seenthat the width of the gap may be increased by the slot 86 by a factor offrom live to ten times without a material reduction in thecross-sectional area of the pole members 81e and 82e. This expedientserves to concentrate the ux across the gap 4S at the lower surface ofthe pole structure, particularly near the outer end thereof. This tendsto flatten out the characteristic curve of the recording head, sinceonly that portion of the flux which crosses the gap near theundersurface of the structure is effective in magnetizing the recordingl medium.

Figure 13 represents a typical test record produced by the operation ofthe apparatus thus far described. Figure 13 comprises a visible recordof the frequency response characteristic of an amplifier. Such a recordmay be produced by moving the recording medium 32 past the recordinghead 33 at a constant speed while there `is applied to the input of theamplifier under test a signal of constant amplitude whose frequency iscaused to vary with time from a very low frequency to a very highfrequency. The output of the amplifier is connected to the electromagnet40 of the recording head, and the voltage applied to the electromagnetwill vary as the amplification of the amplier varies as a function ofthe applied frequency.

The magnetic record thus produced reveals, when treated with the visiblemagnetic material as hereinbefore described, a curve of the charactershown in Figure 13 and representing diagrammatically the frequencyresponse characteristic of the amplifier. As is shown in Figure 13, thelow frequency signals represented by the halfcycle peaks 70,71, 72 etc.is indicated as having been somewhat attenuated by the amplifier undertest. A relatively flat region 73 is shown at frequencies of anintermediate range with a maximum gain being revealed at the upper end74 of the middle frequency range. A rather sharp cut-off 7S at the highfrequency end of the spectrum is revealed.

Figure 14 represents diagrammatically the type of visible recordproduced by a modified recording head 33a represented in Figure 15. Therecording head 33a is identical to that illustrated in Figure 2, exceptfor providing a second pole structure 41', 4Z' identical to the polestructures 41, 42 but extending outwardly to the rear from the magneticcircuit members 43, 44. The pole structures are, of course, madesomewhat shorter than in the form shown in Figure 2 if the device is tobe used with a recording medium 32 of the same width.

It will be appreciated that with the structure shown in l0 Figure 15,when the energy applied to the electromagnet 40 is increased the firstrecording is made at the center of the recording medium where the twopole structures 41, 42 and 41' and 42 are joined t-o each other and tothe magnetic circuit members43, 44. This point defines a neutral axiswhich is represented in Figure 14 bythe broken line 90. As the signallevel is increased, the length of the line permanently magnetizedincreases outwardly in both directions from the neutral axis 90.

Thus, when a signal of the character represented in Figure 8 is appliedto the recording head shown in Figure 15, there results a record Aof thetype shown in Figure 14 wherein the positive peaks 64 and 65 arereproduced and `are accompanied by mirror images 64 and 65 thereof.Similarly, the negative peaks 66 and 67 are recorded as well as thepositive mirror images 66 and 67'.

The devices herein described employ pole structures comprising a pair ofpole members wherein each pole member comprises a single homogeneousmember of suitably high reluctance per unit length. Such a suitably highreluctance may be obtained through appropriate selection of material.Such a member may, for example, be formed of a suitable synthetic resinheavily impregnated with powdered magnetic material such as iron dust.Somewhat similar structures can be made from powdered magnetic materialswhich are bonded to each other by a suitable adhesive or by means of asintering or semisintering process.

' In using any of the recording heads hereinbefore described, the polemembers are preferably held lightly in contact with the surface of therecording medium, and disposed with the length of the pole membersextending transversely of the direction of movement of the record,

ing medium. Also, to prevent local demagnetization and to concentratethe effective magnetizing force in the region of the recording gap, thepole structures illustrated in each of Figures 2, 9, 10, ll, 12, and l5are preferably formed with a partially cylindrical or downwardly convexsurface, or, alternatively, the recording medium is drawn over anoppositely curved surface, so as to limit the area of contact of thepole members with the recording medium to a narrow band in the immediatevicinity of the recording gap.

From the foregoing, it will be observed that this in-v vention providesan apparatus for producing a variable area magnetic recording, and thatthe apparatus includes a variable area magnetic recording headcharacterized by the employment of a recording gap of uniform width andsubstantial length as distinguished from the disclosures contained inthe aforementioned copending application Serial No. 250,364 wherein thegap is given a V-shape, or wherein the recording pole pieces are spacedvariously from the surface of the recording medium. The inventiondescribed herein avoids certain of the disadvantages inherent in theapparatus disclosed in that copending application, and provides varioustypes of structures which are susceptible to ready and accuratemanufacture. Furthermore, as will have been observed, the constructionis such as to permit the shape of the recording characteristic curve tobe selected substantially at will. This is accomplished by appropriatelyselecting the reluctances of the various parts of the magnetic circuit,and does not require any changes in the coniiguration or dimensions ofthe recording gap.

While an attempt has been made herein to disclose and explain theunderlying principles of the invention, no attempt has been made to giveprecise directions for the determination of the dimensions of thevarious parts of the magnetic apparatus for the reason that experiencehas indicated that these dimensions and characteristics are more quicklydetermined by trial and error than by calculation since it is notpossible in calculations of this suicient clarity to permitone skilledin the art to make, without ditculty, a variable area magnetic recordinghead ofthe type herein described, and to permit such a head to be usedfor the making of variable area recordings. It will be observed thatthisinvention provides. variable area magnetic recording heads formaking various types of' records, such as are illustrated for example inFigures 7, 13, and 14. In this connection it is to be notedthat adouble-ended head such as isshown in Figure 15 for producing records ofthe character represented by Figure 14 may utilize pole members 81, 82and S1', 82 lof any of the types and incorporate, as desired, any of thefeatures. of construction previously described with reference to Figures2V to l2, inclusive.

Attention is directed particularly to the fact that by the use of theapparatus herein disclosed, it is possible t'o make visible recordsconstituting oscillograms of the signal applied to the recording head,and that these oscillograms may be rendered immediately visible forstudy without the delay and difcult processing attendant upon the' useof photographic recording methods.

While the various preferred embodiments-of this inventionY have beenillustrated and described herein, the invention'is not to be limited tothe details illustrated and described, except as defined in the appendedclaims.

We claim:

l'. lnA a variable area magnetic recording head for producing on alongitudinally moving elongated magnetizable` recording medium apermanent magnetic record of an electrical signal' in 'which the heightof said record transversely of said medium varies in correspondence withvariations of the instantaneous amplitude of said signal, thecombination of: an electrornagnetY for producing a magnetomotive forcecorresponding to said signals; a pair of magnetic conductors of lowreluctance extendingffrom the ends of said electromagnet topointsadjacentv saidmedium and substantially aligned in a direction parallelto said longitudinal movement of said medium; anda pair of magnetic polestructures of relatively high reluctance andof a length substantiallyequal to the maximumfwidth of the magnetic record disposed in closelyspaced side-by-side relation to define therebetween a narrowrecordinggap of uniform width, one surface of each of said pole structuresadjacent said gap being in contact with said recording medium, said polestructures and the length of said gap therebetween extendingtranserselyof said recording medium, the ends of said inagnetic conductors beingconnected to said pole structures at points spaced from one end of saidstructures, whereby said recording medium is magnetized along saidlength of said gap to a varying degree dependent upon the ampli- 12 tudeof said' signal and the distance from the connection of said magneticconductors to said pole structures;

2. A variable area magneticv recording head accordiltg` to claimV lwherein the reluctance per unit length of each of said magnetic polemembers is uniform from end to end thereof.

3. A variable area magnetic recording head according to claim l in whichsaid magnetic pole members are identical andl of uniform cross-sectionalarea from one end tothe other thereof.

4. A variable area magnetic recording head according to claim l whichincludes a second pair of magnetic pole members identical toy said'first-mentioned pair of magnetic pole members and disposed in axialalignment therewith, and connected in like manner to said pair ofmagnetic conductors, said two pairs of magnetic pole members ex-`tending outwardly in opposite directions from said pair of magneticconductors.

5. .A variable area magnetic recording head according? to'claim 1wherein said adjacently disposed magneticl polemembers are provided witha groove extending downwardly from the upper surface thereof andextending.'

longitudinally of said= pole members in coplanar relationship to saidrecording gap, said groove having a widthf a number ofV times greaterthan the width ofsaidy recording gap and having a depth whichl Variesprogressively from one end of said pole members to the other endthereof.

6. A variable area magnetic recording head according tofclaim 1 inwhich'` at least one of said magnetic poley membersy is longitudinallytapered to provide a cross-sectional area which varies progressivelyfrom one end of said member to the other end thereof.

7. A variable'area magnetic recording head according to'y claim lywherein at least one ofsaid magnetic pole members is provided with aWidth dimension normal to" the plane.Y of-'said recordingv gap whichvaries progressively from one end of said pole member to the othery endthereof.

8.- A variablerarea magnetic recording head according- References Citedin the le of this patent UNITED STATES PATENTS 2,546,850 ChancenotteMar. 27, 1951 2,594,414 Garleau Apr. 29, 1952 2,632,061 Begun a Mar. l7,1953

